This invention relates to an electrospark machining control device, and more particularly to one which controls a discharge gap with a servo system having sample, logic operation and hold functions.
In general, the servo system of an electric discharge machine should respond to an inter-electrode gap at the high speed, according to electric discharge conditions. For this purpose, servo actuators utilizing a hydraulic cylinder, a pulse motor or a DC servo motor have been used to drive the electrode, and a servo system having sample, logic operation and hold functions has been tried for controlling the inter-electrode gap. In the servo system, a general-purpose processing unit such as a micro-processor or a minicomputer can be used.
FIG. 1 shows one example of a conventional control device of this type, in which a DC servo motor is employed as an actuator 10. A pulse voltage, which is subjected to current control, is applied to an inter-electrode gap between an electrode 12 and the workpiece 14 from a pulse source 16. The inter-electrode voltage is digitalized by an analog-to-digital converter 18 and is then applied to a device 20 having the sample, logic operation and hold functions. In the device 20, the inter-electrode voltage is sampled by an input latch circuit 22 at predetermined intervals of time .DELTA.t. The inter-electrode voltage thus sampled is converted into a main shaft position instruction value by a logic operation circuit 24. This instruction value is held by an output latch circuit 26 until the next instruction value is provided. The instruction value is converted from a parallel digital output into a series digital output by a binary rate multiplier (BRM) 28, and is applied, as a count input, to an error counter 30 forming an electrode position servo system. In the counter 30, the amount of main shaft movement which is detected by a main shaft position detector 44, such as a linear encoder magnet scale, is subtracted from the instruction value. The result of this subtraction, namely, a position error signal, is converted into an analog signal by a digital-to-analog converter 32. The analog signal is applied, as a speed instruction signal, to a drive speed control servo system for the electrode 12, which includes a speed amplifier 34, a DC servo motor 36 and pilot generator 38, so that the DC servo motor 36 is rotated, to turn a drilling screw or worm gear 40, to thereby linearly move the main shaft 42 to which the electrode 12 is fixed. Thus, the DC servo motor 36 rotate until the displacement of the main shaft 42 becomes equal to the position instruction, thus controlling the inter-electrode gap.
The above-described control device has heretofore been used for position control according to numerical control (NC); that is, it is used to perform the so-called "variable value control". Therefore, the response speed of the position servo control thereof is on the order of 20 KHz at maximum, because in the conventional variable value control, accuracy in the static condition is essential, and therefore the higher response speeds are unnecessary. Therefore, as shown in FIG. 3, with respect to a ramp-shaped position instruction a from the BRM 28, the main shaft 42 follows a characteristic curve b, and accordingly with respect to the desired position Z.sub.1 a droop .DELTA.Z occurs at the point p, as measured at time t1.
On the other hand, in electrospark machining, follow-up control is effected with respect to the conditions of the inter-electrode gap, and therefore a response speed of 50 KHz at minimum is required for stable electrospark machining. Therefore, although the conventional control device as shown in FIG. 1 can position the electrode with high accuracy in the static condition, it is too inferior in response speed to be practically employed as a servo system under the dynamic conditions described above. In this respect, the above-described conventional control system is deficient.